1. Field of the Invention
The present invention relates to an internal touch contact for an integrated circuit (IC) socket, and particularly to a soft internal touch contact for a land grid array (LGA) socket for facilitating signal transmission between an IC package and a printed circuit board (PCB).
2. Description of Prior Art
IC packages having leads arranged in a land grid array (LGA) are well known as LGA packages. The LGA packages have a relatively low height, which is desirable for saving space in electronic assemblies.
Connectors for removably mounting an LGA package on a PCB are known as LGA sockets. An LGA socket typically comprises a substantially flat dielectric housing which is positioned between the LGA package and the PCB. The housing defines an array of passageways with electrical contacts received therein in correspondence with the array of leads of the LGA package. Each contact has a pair of oppositely extending free ends spaced apart a predetermined distance in an original position. The two free ends project beyond external surfaces of the socket housing for respectively engaging with corresponding contact pads on a bottom surface of the LGA package and on a top surface of the PCB. When the LGA package is sandwiched between the socket and the PCB, the two free ends of each contact are compressed to contact each other thereby shortening the signal transmission path. Typically, the compressive force may be applied by pressure plates which are fastened together to sandwich the package, the socket and the PCB therebetween. Even if the two free ends of the contact fail to contact each other, the signal can still be transmitted through the contact but along a relatively longer path. This kind of contact is called an internal touch contact.
Various internal touch contacts for LGA sockets are disclosed in the prior art. One problem encountered with the conventional internal touch contacts, such as those disclosed in U.S. Pat. Nos. 4,262,986; 4,268,102 and 5,092,783, is that one of the two free ends of the contact is constructed as a flat, non-yielding part whereby the normal force required to establish the contact interface is increased so sharply that it has proven difficult to control it within desired limits. The increased normal force generally has a negative effect on connector performance. Therefore, it is desired to have a soft internal touch contact that operates using a small normal force.
U.S. Pat. Nos. 4,354,729; 4,511,197 and 4,969,826 disclose internal touch contacts having free ends thereof engaged with each other via side edges thereof. As such contacts become increasingly miniaturized, and as the thickness of the contacts is decreased, it becomes increasingly difficult to mate the side edges of the two free ends of the contact. Hence, a contact having free ends thereof engaged with each other via major surfaces thereof is also required to ensure reliable internal contact performance.
Furthermore, the conventional LGA sockets do not provide solutions to CTE (Coefficient of Thermal Expansion) problems. During cooling after the contacts are soldered to corresponding contact pads of the PCB, stresses resulting from a difference between the CTEs of the socket housing and the PCB will develop in the soldered connections between the contact and the PCB, possibly causing failure of soldered connections. Additionally, when there is a large change in environmental temperature, larger stresses can develop in the soldered connections. Accordingly, an improved LGA socket is required to overcome the above disadvantages of conventional LGA sockets.